SKPhysicsJoints are all fairly wobbly and elastic

Question

So, I have an object composed of several physics bodies with 10 or so joints holding it together (pin and fixed). It works well but I am surprised at how wobbly the joints are. I'd imagine "fixed" to be just as fixed as if I merged them into one physics body, but it's substantially wobbly to the point that it is unusable. Also with pins, the points on each physics body seem to have quite some play apart from each other when force is applied to connecting objects. This is the case all the time, and both bodies involved are not restricted in motion in any way.

I would love to have the joints more rigid and less elastic. Has anyone been able to find a solution or reason for this? adding several more joints does not seem to make it any more rigid.

Answer 1

I faced a similar elasticity bug with a rope simulation and could finally come up with a workaround.

Here's my rope interface:

#import <SpriteKit/SpriteKit.h>

@interface ALRope : NSObject

@property(nonatomic, readonly) NSArray *ropeRings;

@property(nonatomic) int ringCount;

@property(nonatomic) CGFloat ringScale;

@property(nonatomic) CGFloat ringsDistance;

@property(nonatomic) CGFloat jointsFrictionTorque;

@property(nonatomic) CGFloat ringsZPosition;

@property(nonatomic) CGPoint startRingPosition;

@property(nonatomic) CGFloat ringFriction;

@property(nonatomic) CGFloat ringRestitution;

@property(nonatomic) CGFloat ringMass;


@property(nonatomic) BOOL shouldEnableJointsAngleLimits;

@property(nonatomic) CGFloat jointsLowerAngleLimit;

@property(nonatomic) CGFloat jointsUpperAngleLimit;



-(instancetype)initWithRingTexture:(SKTexture *)ringTexture;


-(void)buildRopeWithScene:(SKScene *)scene;

-(void)adjustRingPositions;

-(SKSpriteNode *)startRing;

-(SKSpriteNode *)lastRing;

@end

Rope Implementation:

#import "ALRope.h"

@implementation ALRope

{
    SKTexture *_ringTexture;

    NSMutableArray *_ropeRings;
}

static CGFloat const RINGS_DISTANCE_DEFAULT = 0;

static CGFloat const JOINTS_FRICTION_TORQUE_DEFAULT = 0;

static CGFloat const RING_SCALE_DEFAULT = 1;

static int const RING_COUNT_DEFAULT = 30;

static CGFloat const RINGS_Z_POSITION_DEFAULT = 1;

static BOOL const SHOULD_ENABLE_JOINTS_ANGLE_LIMITS_DEFAULT = NO;

static CGFloat const JOINT_LOWER_ANGLE_LIMIT_DEFAULT = -M_PI / 3;

static CGFloat const JOINT_UPPER_ANGLE_LIMIT_DEFAULT = M_PI / 3;

static CGFloat const RING_FRICTION_DEFAULT = 0;

static CGFloat const RING_RESTITUTION_DEFAULT = 0;

static CGFloat const RING_MASS_DEFAULT = -1;


-(instancetype)initWithRingTexture:(SKTexture *)ringTexture
{
    if(self = [super init]) {
        _ringTexture = ringTexture;

        //apply defaults
        _startRingPosition = CGPointMake(0, 0);
        _ringsDistance = RINGS_DISTANCE_DEFAULT;
        _jointsFrictionTorque = JOINTS_FRICTION_TORQUE_DEFAULT;
        _ringScale = RING_SCALE_DEFAULT;
        _ringCount = RING_COUNT_DEFAULT;
        _ringsZPosition = RINGS_Z_POSITION_DEFAULT;
        _shouldEnableJointsAngleLimits = SHOULD_ENABLE_JOINTS_ANGLE_LIMITS_DEFAULT;
        _jointsLowerAngleLimit = JOINT_LOWER_ANGLE_LIMIT_DEFAULT;
        _jointsUpperAngleLimit = JOINT_UPPER_ANGLE_LIMIT_DEFAULT;
        _ringFriction = RING_FRICTION_DEFAULT;
        _ringRestitution = RING_RESTITUTION_DEFAULT;
        _ringMass = RING_MASS_DEFAULT;
    }
    return self;
}


-(void)buildRopeWithScene:(SKScene *)scene
{
    _ropeRings = [NSMutableArray new];
    SKSpriteNode *firstRing = [self addRopeRingWithPosition:_startRingPosition underScene:scene];

    SKSpriteNode *lastRing = firstRing;
    CGPoint position;
    for (int i = 1; i < _ringCount; i++) {
        position = CGPointMake(lastRing.position.x, lastRing.position.y - lastRing.size.height - _ringsDistance);
        lastRing = [self addRopeRingWithPosition:position underScene:scene];
    }

    [self addJointsWithScene:scene];
}

-(SKSpriteNode *)addRopeRingWithPosition:(CGPoint)position underScene:(SKScene *)scene
{
    SKSpriteNode *ring = [SKSpriteNode spriteNodeWithTexture:_ringTexture];
    ring.xScale = ring.yScale = _ringScale;
    ring.position = position;
    ring.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:ring.size.height / 2];
    ring.physicsBody.allowsRotation = YES;
    ring.physicsBody.friction = _ringFriction;
    ring.physicsBody.restitution = _ringRestitution;
    if(_ringMass > 0) {
        ring.physicsBody.mass = _ringMass;
    }

    [scene addChild:ring];
    [_ropeRings addObject:ring];
    return ring;
}

-(void)addJointsWithScene:(SKScene *)scene
{
    for (int i = 1; i < _ropeRings.count; i++) {
        SKSpriteNode *nodeA = [_ropeRings objectAtIndex:i-1];
        SKSpriteNode *nodeB = [_ropeRings objectAtIndex:i];
        SKPhysicsJointPin *joint = [SKPhysicsJointPin jointWithBodyA:nodeA.physicsBody
                                                               bodyB:nodeB.physicsBody
                                                              anchor:CGPointMake(nodeA.position.x,
                                                                                 nodeA.position.y - (nodeA.size.height + _ringsDistance) / 2)];
        joint.frictionTorque = _jointsFrictionTorque;
        joint.shouldEnableLimits = _shouldEnableJointsAngleLimits;
        if(_shouldEnableJointsAngleLimits) {
            joint.lowerAngleLimit = _jointsLowerAngleLimit;
            joint.upperAngleLimit = _jointsUpperAngleLimit;
        }
        [scene.physicsWorld addJoint:joint];
    }
}

//workaround for elastic effect should be called from didSimulatePhysics
-(void)adjustRingPositions
{
    //based on zRotations of all rings and the position of start ring adjust the rest of the rings positions starting from top to bottom
    for (int i = 1; i < _ropeRings.count; i++) {
        SKSpriteNode *nodeA = [_ropeRings objectAtIndex:i-1];
        SKSpriteNode *nodeB = [_ropeRings objectAtIndex:i];
        CGFloat thetaA = nodeA.zRotation - M_PI / 2,
        thetaB = nodeB.zRotation + M_PI / 2,
        jointRadius = (_ringsDistance + nodeA.size.height) / 2,
        xJoint = jointRadius * cosf(thetaA) + nodeA.position.x,
        yJoint = jointRadius * sinf(thetaA) + nodeA.position.y,
        theta = thetaB - M_PI,
        xB = jointRadius * cosf(theta) + xJoint,
        yB = jointRadius * sinf(theta) + yJoint;
        nodeB.position = CGPointMake(xB, yB);
    }
}

-(SKSpriteNode *)startRing
{
    return _ropeRings[0];
}

-(SKSpriteNode *)lastRing
{
    return [_ropeRings lastObject];
}

@end

Scene code to showcase how to use the Rope:

#import "ALRopeDemoScene.h"
#import "ALRope.h"

@implementation ALRopeDemoScene
{
    __weak SKSpriteNode *_branch;

    CGPoint _touchLastPosition;

    BOOL _branchIsMoving;

    ALRope *_rope;
}

-(id)initWithSize:(CGSize)size {
    if (self = [super initWithSize:size]) {
        /* Setup your scene here */

        self.backgroundColor = [SKColor colorWithRed:0.2 green:0.5 blue:0.6 alpha:1.0];

        [self buildScene];
    }
    return self;
}

-(void) buildScene {
    SKSpriteNode *branch = [SKSpriteNode spriteNodeWithImageNamed:@"Branch"];
    _branch = branch;
    branch.position = CGPointMake(CGRectGetMaxX(self.frame) - branch.size.width / 2,
                                  CGRectGetMidY(self.frame) + 200);
    branch.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(2, 10)];
    branch.physicsBody.dynamic = NO;
    [self addChild:branch];
    _rope = [[ALRope alloc] initWithRingTexture:[SKTexture textureWithImageNamed:@"rope_ring"]];

    //configure rope params if needed
    //    _rope.ringCount = ...;//default is 30
    //    _rope.ringScale = ...;//default is 1
    //    _rope.ringsDistance = ...;//default is 0
    //    _rope.jointsFrictionTorque = ...;//default is 0
    //    _rope.ringsZPosition = ...;//default is 1
    //    _rope.ringFriction = ...;//default is 0
    //    _rope.ringRestitution = ...;//default is 0
    //    _rope.ringMass = ...;//ignored unless mass > 0; default -1
    //    _rope.shouldEnableJointsAngleLimits = ...;//default is NO
    //    _rope.jointsLowerAngleLimit = ...;//default is -M_PI/3
    //    _rope.jointsUpperAngleLimit = ...;//default is M_PI/3

    _rope.startRingPosition = CGPointMake(branch.position.x - 100, branch.position.y);//default is (0, 0)

    [_rope buildRopeWithScene:self];

    //attach rope to branch
    SKSpriteNode *startRing = [_rope startRing];
    CGPoint jointAnchor = CGPointMake(startRing.position.x, startRing.position.y + startRing.size.height / 2);
    SKPhysicsJointPin *joint = [SKPhysicsJointPin jointWithBodyA:branch.physicsBody bodyB:startRing.physicsBody anchor:jointAnchor];
    [self.physicsWorld addJoint:joint];
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint location = [touch locationInNode:self];
    if(CGRectContainsPoint(_branch.frame, location)) {
        _branchIsMoving = YES;
        _touchLastPosition = location;
    }
}

-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    _branchIsMoving = NO;

}

-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

    if(_branchIsMoving) {
        UITouch *touch = [touches anyObject];
        CGPoint location = [touch locationInNode:self],
        branchCurrentPosition = _branch.position;
        CGFloat dx = location.x - _touchLastPosition.x,
        dy = location.y - _touchLastPosition.y;
        _branch.position = CGPointMake(branchCurrentPosition.x + dx, branchCurrentPosition.y + dy);
        _touchLastPosition = location;
    }
}

-(void)didSimulatePhysics
{
    //workaround for elastic effect
    [_rope adjustRingPositions];
}


@end

Notice the [rope adjustRingPositions] call from [scene didSimulatePhysics]. That was the actual workaround for the elastic bug.

Complete demo code is here. I hope this helps!